In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high device densities there have been, and continue to be, efforts toward scaling down device dimensions (e.g., at sub-micron levels) on semiconductor wafers. In order to accomplish such densities, smaller feature sizes and more precise feature shapes are required. This may include width and spacing of interconnecting lines, spacing and diameter of contact holes, and surface geometry, such as corners and edges, of various features. The dimensions of and between such small features can be referred to as critical dimensions (CDs). Reducing CDs and reproducing more accurate CDs facilitates achieving higher device densities.
High resolution lithographic processes are used to achieve small features. In general, lithography refers to processes for pattern transfer between various media. In lithography for integrated circuit fabrication, a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the photoresist. The film is selectively exposed with radiation (e.g., optical light, x-ray, electron beam, . . . ) through an intervening master template (e.g., mask, reticle, . . . ) forming a particular pattern (e.g., patterned resist). Dependent upon coating type, exposed areas of the coating become either more or less soluble than unexposed areas in a particular solvent developer. More soluble areas are removed with the developer in a developing step, while less soluble areas remain on the silicon wafer to form a patterned coating. The pattern corresponds to either the image of the mask or its negative. The patterned resist is used in further processing of the silicon wafer.
Efforts to reduce CDs have included implementing various techniques in connection with the lithographic process, such as reducing exposure radiation wavelength (eg., from 436 nm mercury g-line to 365 nm i-line to 248 nm DUV to 193 nm excimer laser), improving optical design, utilizing metrology techniques (e.g., scatterometry, scanning electron microscope (SEM)), etc. Immersion lithography facilitates further reduction of CDs.
In immersion lithography, the gap between a substrate (e.g., wafer) and a final optical component (e.g., lens) is filled with an immersion medium, which has a higher refractive index than air. Refractive index is defined as a ratio of speed of light in a vacuum to speed of light in a particular medium. Utilizing an immersion medium with a refractive index greater than that of air, which approximately equals 1, can increase numerical aperture, which is defined as a lens's ability to gather diffracted light and resolve fine details onto a wafer. Furthermore, utilization of an immersion medium can decrease an effective wavelength of an exposure radiation propagating within the immersion medium without changing exposure radiation, lasers, lens materials, etc.
Currently, immersion lithography is limited by an inability to monitor and control immersion medium properties such as, for example, refractive index (n) and lithography constant (k). Conditions that can impact these properties include, for example, temperature, pressure, formation of microbubbles, chemical contamination of fluid, thermal and mechanical changes, etc. These conditions can impact efficiency of immersion lithography systems and can elevate costs for expensive immersion mediums. Thus, there exists a need in the art for systems and methods that can monitor and/or control immersion medium properties.